This application relates generally to FT-IR (Fourier transform infrared spectroscopy) and more specifically to multiple modulation measurements with a polarization photoelastic modulator (PEM).
A Fourier transform spectrometer typically includes an interferometer into which are directed a beam of analytic radiation (typically an infrared beam) to be analyzed and a monochromatic (laser) beam that provides a position reference. The interferometer has first and second mirrors.
Each of the input beams is split at a beamsplitter with one portion traveling a path that causes it to reflect from the first mirror and another portion traveling a path that causes it to reflect from the second mirror. The portions of each beam recombine at the beamsplitter, and the recombined beams are directed to appropriate detectors. The difference between the optical paths traveled by the first and second portions of the beams is often referred to as the retardation or retardation value.
One of the mirrors (referred to as the fixed mirror) is fixed or movable over a limited range while the other mirror (referred to as the movable mirror) is movable over a much more extensive range. In rapid scanning, the retardation is changed at a nominally constant rate over a significant range. This is typically accomplished by moving the second mirror at a nominally constant velocity. In step scanning, the retardation is changed intermittently, in relatively small steps of retardation. In some implementations, this is accomplished by stepping the movable mirror position.
The optical interference between the two beam portions causes the intensity of the monochromatic beam and each frequency component of the infrared beam to vary as a function of the component's optical frequency and the retardation. The detector output represents the superposition of these components and, when sampled at regular distance intervals, provides an interferogram whose Fourier transform yields the desired spectrum.
The monochromatic beam provides a reference signal whose zero crossings occur each time the relative position between the fixed and movable mirrors changes by an additional one quarter of the reference wavelength (i.e., for each half wavelength change of retardation). The data acquisition electronics are triggered on some or all of these zero crossings to provide regularly sampled values for the interferogram.
In a step-scan interferometer, the relative position between the fixed and movable mirrors is stepped from one retardation value to the next and then held, at which point an intensity measurement is made. The sequence is then repeated until the desired interferogram has been acquired. The prior art teaches various techniques for accomplishing this under servo control. A number of approaches are disclosed in U.S. Pat. No. 5,166,749 [Curbelo92b], which is incorporated by reference in its entirety for all purposes. Curbelo92b discloses an implementation of step scanning where the movable mirror is driven at a constant velocity and the "fixed" mirror is driven, using an actuator such as a piezoelectric transducer (PZT), in a sawtooth fashion over a small distance corresponding to the desired step size. The superposition of the two movements results in a stepped retardation.
In FT-IR, a PEM is used to modulate the polarization of the spectrometer beam at a PEM drive frequency f.sub.PEM, to allow the measurement of the difference in the spectral characteristic of the sample to different polarizations ([Noda88], [Hinds88]). A PEM circular dichroism (CD) measurement provides the spectral differential absorption of the sample for left and right circular polarized radiation. The desired signal is obtained by demodulating the spectrometer signal at the PEM drive frequency [Griffiths86]. A PEM linear dichroism (LD) measurement provides the spectral differential absorption of the sample for different linear polarizations, with demodulation at twice the PEM drive frequency. A dynamic infrared linear dichroism (DIRLD) measurement provides the effect on the sample linear dichroism from a dynamic strain modulation at a sample modulation frequency f.sub.Sample [Noda88].
In multiple modulation measurements, the data processing system until recently used multiple lock-in amplifiers (LIAs) . However, digital signal processing (DSP) techniques are not new in FT-IR [Manning93], and DSP techniques have been used for apodization, Fourier transform, phase correction, and some digital filtering. In a typical DSP solution, all the modulation drive signals are derived from a system master clock, thereby making it possible to demodulate the detector signal's different frequencies using similarly derived signals.
However, PEM measurements present a special problem that suggests that a DSP solution would be unsuitable. In particular, demodulating the PEM carrier signal with a DSP process would require knowing not only the exact frequency of the PEM drive at the time the data was collected, but also its phase relative to the sampling clock. Unfortunately, locking the PEM drive to the system master clock is not practical, as the bandwidth of the PEM is comparable to its resonant frequency drift, and would result in a changing PEM modulation index during a measurement.